The blue dye indigo, produced by some plants, is one of the oldest dyes in the world. It is still a widely used textile dye, particularly in the denim clothing industry. Modern indigo is no longer sourced from plants, but is produced synthetically, mostly from petroleum. Indigo itself is a water-insoluble compound. In order to dye denim, indigo must be treated with a reducing agent to produce an unstable, water-soluble intermediate. After application of this intermediate to the fabric, the fabric is exposed to air and the intermediate oxidizes back to indigo, crystallizing within the fabric fibers. The most common reducing agent used for this process is sodium dithionite, which has many limitations. Sodium dithionite is unstable; can over-reduce the indigo, destroying the dye; is required in excess quantities; and its use produces large amounts of sulfate and sulfite which are detrimental to the environment. Because of these shortcomings, substantial amounts of sodium dithionite are lost to byproducts and degradation during the dyeing process.
The biosynthesis of natural indigo in plants proceeds through a different synthetic pathway. An indigo precursor compound is produced and stored in the plant, then converted to indoxyl, which spontaneously dimerizes to form indigo. However, the quantity and variety of indigo precursors that can be produced through current natural biological pathways is limited.
Thus, there exists a need for new methods to produce indigoid dye precursors with greater structural variety and which can be used to produce indigoid dyes without a harsh reductant chemical.